In the metal industry, it has been common for a period of time to use disposable thermocouples for measuring the temperature of a molten bath and to use disposable thermocouples as part of a phase change device. Such phase change devices are utilized, for example, when detecting the thermal arrest temperature of a molten metal.
Examples of prior art related to disposable thermocouples for use in connection with measuring the temperature of a molten bath include U.S. Pat. Nos. 3,011,005; 3,048,642; 3,493,439; and 3,531,331. Examples of prior art disclosing phase change cups which include a thermocouple are 3,267,732, 3,611,808 and 3,946,594.
Difficulties have been noted, especially in connection with phase change cups, with respect to the appreciable length of time during which the thermocouple hot junction is subjected to considerable stresses. It is often noted that temperature measurements are not perfectly reproducible from one device to the next. It is believed that the discrepancy is due mainly to the manner of forming the thermocouple hot junction on the one hand, and due to the influence of carbonization of resins in the refractory cup which becomes slightly electrically conductive. The present invention overcomes each of these objectionable features.
The thermocouple assembly of the present invention includes dissimilar metals joined at a hot junction within a protective annular shield which is transparent to radiation. The hot junction is formed in situ so as to be in intimate contact with substantially 360.degree. of the inner circumference of said shield. When the thermocouple assembly is applied to a cup of the phase change type, one of the dissimilar metals is electrically insulated from contact with the refractory material from which the cup is made.
Heretofore, it has been conventional to form a hot junction between dissimilar metals for use as a disposable thermocouple by welding and/or twisting together adjacent ends of the dissimilar metal wires. Thereafter, the metal wires are inserted into a straight or U-shaped protective shield from a material such as quartz in a manner so that one end of each of the metal wires projects from the opposite ends of the shield. When the shield is U-shaped, the hot junction is positioned adjacent to the bight of the shield. The thermocouple assembly of the present invention is distinguished from such conventional procedure particularly by forming the hot junction of the juxtaposed ends of the thermocouple wires in situ within the protective shield.
It is an object of the present invention to provide a method of thermocouple assembly which produces more uniform results.
It is another object of the present invention to provide a method of constructing a phase change device which produces more uniform results.
Other objects will appear hereinafter.